The invention is based on a spark plug for an internal combustion engine and a method for producing a middle electrode as generically defined by the preambles to the independent claims. A spark plug for an internal combustion engine is already known (EP 0 785 604 B1), which has a middle electrode comprised of an electrode base body and a precious metal platelet. The precious metal platelet is attached to the end face of the electrode base body oriented toward the combustion chamber. In its end section oriented toward the combustion chamber, the electrode base body is the shape of a truncated cone. EP 0 785 604 B1 has also disclosed affixing the precious metal platelet to the end face of the electrode base body oriented toward the combustion chamber by means of laser welding or resistance welding. The precious metal platelet is comprised of a platinum-, iridium-, or platinum-based alloy. The electrode base body is comprised of a nickel alloy and has a core made of thermoconducting material.
The spark plug for an internal combustion engine according to the invention, with the features of the independent claim, has the advantage over the prior art that it has very favorable ignition properties since a reduced surface area reduces the amount of heat drawn away from the volume in which the mixture is to be ignited. An embodiment of this kind is inexpensive to produce.
Advantageous modifications and improvements of the spark plug disclosed in the independent claim are possible by means of measures taken in the dependent claims. It is particularly advantageous to select the opening angle of the truncated cone-shaped precious metal platelet as smaller than the opening angle of the truncated cone-shaped combustion chamber end section of the electrode base body since this minimizes material consumption for the precious metal platelet and at the same time minimizes the removal of heat from the volume in which the combustible mixture is to be ignited. It is particularly advantageous to embody the combustion chamber end section of the electrode base body in such a way that it has a first and a second truncated cone-shaped region. This further reduces the removal of heat from the volume in which the combustible mixture is to be ignited. For the correct adaptation of the opening angle to the bottom diameter of the electrode base body, it is advantageous to embody the opening angles in such a way that the opening angle of the first truncated cone-shaped region and the precious metal platelet connected to it is smaller than the opening angle of the second truncated cone-shaped region. Furthermore, it is advantageous to enlarge the combustion-resistant precious metal region, i.e. to embody the first truncated cone-shaped region and the region of the precious metal platelet adjoining it on the side oriented toward the combustion chamber in such a way that the opening angle of the first truncated cone-shaped region points toward the combustion chamber. This truncated cone-shaped region therefore widens out slightly, i.e. at an angle of up to 35xc2x0, in the direction toward the combustion chamber. Consequently, the removal of heat from the region in which the combustible mixture is to be ignited is thus not significantly increased, but an increase in the combustion resistance is achieved.
The method according to the invention for producing a middle electrode for a spark plug of an internal combustion engine, with the features of the independent claim, has the advantage over the prior art that the adhesion of the precious metal tip to the supporting material of the electrode base body is improved. Advantageous modifications and improvements of the method disclosed in the independent claim for producing a middle electrode for a spark plug of an internal combustion engine are possible by means of the measures taken in the dependent claims. It is particularly advantageous to affix the precious metal platelet to the electrode base body by means of a simple method such as resistance welding or laser welding. It is also advantageous that the combustion chamber end face of the electric base body is machined in a material-removing manner before the attachment of the precious metal platelet in such a way that the end face is flat since this results in a precisely defined attachment of the platelet and consequently a precise localization of the region between the precious metal platelet and the electrode base body. It is also advantageous to execute the material-removing machining of the precious metal platelet and the end section of the electrode base body in such a way that the combustion chamber end section of the electrode base body has a first and a second truncated cone-shaped region. On the one hand, this assures a favorable adhesion of the precious metal platelet and on the other hand, assures that the heat-dissipating surface area of the middle electrode is minimized.